July 31, 2014

Air Independent Propulsion - A Game Changer?

Hydrogen-oxygen fuel cell system. (Diagram courtesy of  http://webberswarships.ca/styled-9/index.html )

MESMA closed-cycle steam turbine

Stirling-cycle heat engine with external combustion

Closed-cycle diesel engine? (Diagram courtesy of http://webberswarships.ca/styled-9/index.html )

Diesel-electric engine for submarine, which can use any of the AIP technologies above.

Descriptions of the strengths and weaknesses of each AIP technology is on this website at Air independent propulsion (AIP) Technologies and Selection

If-when Australia chooses an air independent propulsion (AIP) system for the long awaited Future Submarine Australia will have several technologies (above) to decide on. Perhaps Australia might choose no AIP if Lithium-ion battery technology is considered adequate.

The following is an excellent article, dated January 29, 2013, by Michael Raska, a Research Fellow at the Institute of Defence and Strategic Studies, S. Rajaratnam School of International Studies (RSIS), Nanyang Technological University in Singapore. It has been republished by Eurasia Reviewhttp://www.eurasiareview.com/29012013-submarine-trends-in-asia-pacific-air-independent-propulsion-a-game-changer-analysis/ :


JANUARY 29, 2013

The contending strategic realities of the Asia-Pacific region compel states to adopt innovations of their rivals. This is the case for new classes of conventional submarine designs, which incorporate an array of innovative technologies in order to maximise their survivability and lethality in diverse maritime operations.
By Michael Raska
WHILE EUROPE and North America remain key submarine markets, China’s ongoing military modernisation coupled with contending international relations in the Asia-Pacific will increasingly drive submarine procurement in the region over the next decade. In 2011, the total submarine market in Asia-Pacific is estimated at US$4.4 billion, and for the next decade, submarine expenditures are projected to US$46 billion. 
With changing strategic realities, Asian navies aim to become increasingly flexible, and capable of varying mission profiles: from countering traditional coastal defence missions to protecting sea lanes and communication lines. Simultaneously, submarines are increasingly valuable strategic resource for both electronic and signal intelligence. To enhance the varying operational capabilities, increase submerged endurance and stealth, installing viable Air-independent propulsion systems is thus becoming a strategic necessity.
Advantages of AIP systems
Designed to enhance the performance of modern conventional (diesel-electric) submarines AIP is a key emerging technology that essentially provides a “closed cycle” operation through a low-power electrical source supplementing the battery, which may extend the submarine’s underwater endurance up to two weeks or more.
AIP systems close the endurance gap between nuclear and conventional submarines, and mitigate increasing risks of detection caused by advanced anti-submarine warfare technologies – from modern electro-optical systems and surface radars to magnetic sensors, active and passive sonars, and airborne surveillance radars. Advanced AIP technologies thus promise significant operational advantages and tactical flexibility.
In theory, there are four primary AIP designs currently available: (1) closed-cycle diesel engines; (2) closed-cycle steam turbines; (3) Stirling-cycle heat engines with external combustion, and (4) hydrogen-oxygen fuel cells. Each provides a different solution with particular advantages as well as limitations in relation to performance, safety, and cost factors.
Since the early years of the Cold War, while major naval powers shifted to nuclear propulsion, smaller navies – particularly in Europe (Germany, Sweden, Spain, Italy and France) continued to develop and rely on conventional diesel-electric submarine fleets, given their lower cost and operational relevance for coastal defence. Traditionally, however, these submarines were highly vulnerable to various types of sensors – acoustic, visual, thermal and air – particularly when running on engines.

AIP systems in Asian navies

On the other hand, when running on batteries, these submarines became very quiet and difficult to detect, yet their battery capacity, discharge rate, and indiscretion rate (the ratio of diesel running time to total running time) substantially limited their underwater endurance. To overcome these baseline limitations, naval innovation in propulsion technologies over the past two decades has shifted toward AIP systems.
There is a variance, however, in the procurement of AIP systems in select Asian navies. For example, the only AIP steam-turbine system currently available is the French “MESMA” (Module d’Energie Sous-Marine Autonome) module, operational on Pakistan Navy’s two Agosta 90-B class submarines.
Swedish-Kockum designed Stirling AIP technology is installed on Singapore Navy’s two Archer–class submarines, and Japan’s new Soryu-class submarines. The Chinese PLA Navy’s Type 041 Yuan and Type 043 Qing class submarines are also reportedly using Stirling technology. Meanwhile, the Republic of Korea Navy has ordered nine Type 214 submarines with German HDW AIP fuel cell technologies. Three first batch models of the new Son Won-Il class had entered service since 2007, and six second batch models will enter service from 2012.

Limitations and constraints

Notwithstanding the diverse AIP technologies, the overall effectiveness of each system will depend on how well it is integrated with other critical systems that ensure optimal submarine functions: power systems, sensors systems, safety systems, navigation systems, command, control, and communication systems, weapons systems, and climate control systems. In this context, any critical failure of an AIP during a combat mission or contested areas will mitigate survivability factors as well as tactical options.
Indeed, each AIP system design comes with an array of technological limitations, vulnerabilities, and risks, particularly in submerged operations – from the specific acoustic signatures produced by select AIP systems in specific operating regimes, to technical vulnerabilities in storing oxidizer/fuel, as well as their maintenance regime. At the same time, new anti-submarine warfare sensor technologies may provide viable AIP countermeasures.
Ultimately, AIP-related technological innovation and breakthroughs may not guarantee operational success – strategy, operational concepts, tactical development, leadership, training, and morale will continue to play as important role as emerging technologies and their operational capabilities.
Michael Raska is a Research Fellow at the Institute of Defence and Strategic Studies, a constituent unit of the S. Rajaratnam School of International Studies (RSIS), Nanyang Technological University in Singapore.


Anonymous said...

Dear Pete,

nice to see a new blog entry about submarines. As usual some comments.

The reduction gear box is omitted on German Type 212/214 submarines. The electric motor is directly connected to the drive shaft.

Michael Raska was not aware of alcohol to replace hydrogen for fuel cell use. An intermediate step is the Spanish solution with Bio-ethanol and a hydrogen reformer. The final step could be direct methanol fuel cells (DMFC) for submarines.
So the problem with small and aggressive hydrogen will be gone.

German Navy prefers a methanol solution because ethanol is drinkable (Advocaat below surface).


Michael Raska said...

Hi Pete,

Thank you for posting my article - I am a reader of your blog, and also learn from the interesting discussions here.

A couple of weeks ago, I had a follow-on article in the Diplomat on the submarine modernization trends in East Asia.



Pete said...

Hi MHalblaub

Thanks for the info.

Actually I located the diagrams separately - so more my "error" than Michael's. In theory hydrogen can be used. But I see that hydrogen explosive volatility has lead to a preference for alcohols that are safer but at the expense of lower energy value.

I'll use this reference in the next article http://www.navy.mil/navydata/cno/n87/usw/issue_13/propulsion.htm :

"The greatest challenge for fuel-cell AIP systems lies in storing the reactants. Although oxygen can be handled with relative safety as LOX, storing hydrogen onboard as a liquid or high-pressure gas is very dangerous. One solution is to carry the hydrogen in metal hydride accumulators, at low pressure and ambient sea temperature. (A metal hydride is a solid compound of hydrogen and metallic alloy, in which individual hydrogen atoms occupy interstitial positions in the host metal's crystalline lattice. By manipulating temperature and pressure, hydrogen gas can be absorbed or released at will.) Another, less efficient, approach is to generate gaseous hydrogen from a stored liquid hydrocarbon such as diesel fuel, kerosene, or methanol. This requires an auxiliary device called a "reformer," in which a mixture of hydrocarbon and water is vaporized and superheated under pressure to yield a mixture of hydrogen and carbon dioxide."

I assume the engine room technicians would be too distracted and merry if they used ethanol :)



Pete said...

Hi Michael Raska

The balance in your AIP article certainly attracted my interest.

Actually I noticed your more recent work http://thediplomat.com/2014/07/submarine-modernization-in-east-asia/ in its form on the RSIS site

I was going to post it on my blog but then decided to post your AIP article first. My thought was to address the technical fundamentals (like AIP) between articles discussing submarine trends in the Asia-Pacific region.

What I plan to write next is an article on the strengths and weaknesses of each AIP type. Following that I'll run http://thediplomat.com/2014/07/submarine-modernization-in-east-asia/

And after that an article concentrating on the latest Chinese submarine trends and developments.



Michael Raska said...

Dear Pete,

I also worked on the AIP comparison for a book chapter. It's a great topic in the context of critical emerging technologies and its strategic implications for East Asia.

While I am not by any means a technical expert, here is what I found for the trade-offs:

Similarly to CCD, the MESMA has a number of operational constrains, most notably the requirement to store and handle the liquid oxygen (LOX). Most importantly, while the MESMA may provide higher power output than other AIP alternatives, its net efficiency might be the lowest (est. 25%) as its rate of oxygen consumption is higher. The MESMA waste combustion system also contains moving parts, which may radiate detectable noise-levels. Ultimately, the maintenance and crew training requirements of the MESMA steam turbine system are higher compared to a typical conventional submarine crew.

In case of Stirling engines, while its generators employ advanced noise-reduction technologies, such as double-elastic mounting with a sound proof hood to minimize noise levels transmitted into the sea, the supporting auxiliary equipment contains moving parts that may limit the submarine’s stealth capability. Furthermore, the Stirling engines operate at a pressure of 20 bars, which limits the submarine’s depth capability to 200 meters, unless a power consuming and potentially noisy exhaust gas pressure intensifier is used. Ultimately, as any other AIP system burning diesel and LOX, the LOX supply will remain the principal constraint to achievable endurance.

As for fuel cells, notwithstanding their relatively high cost, the potential constraint of FC systems are in the safety hazard in the process of storing and replacing onboard hydrogen.

Anonymous said...

Hi Pete,
this may be little of the AIP discussion. Could you check out the B90- Sarov class is that an AIP with
radioisotope thermoelectric generators
Disclaimers I don't know much about this however I just came across this a couple of days back.

Pete said...

Hi Harish

I had a look at http://www.informationdissemination.net/2007/12/russias-not-so-super-secret-special.html finding all the links had timed out.

However I looked at http://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator
finding radioisotope thermoelectric generators are suited and designed for low power generally multi-decade functions that are not near humans (due to need to provide inefficient shielding if people were in a vehicle, like a submarine).

Radioisotope thermoelectric generators appear to provide far too little power even to power the computer systems and air-conditioning of subs let alone power propellers.

However radioisotope thermoelectric generators could perhaps power UUVs particularly ones needing to move slowly, relatively short range then sit on the seafloor or maybe tap an undersea cable.

Undersea sensors to register and communicate shipping-sub movements in choke points (eg. straits and ports) for decades might also be an application and may conceivably be in use by some of the P5 (maybe Russia).



Anonymous said...

Dear Michael Raska,

a read your article again Pete posted here. You wrote an AIP could "extend the submarine’s underwater endurance up to two weeks or more".
The official German record is to travel for 18 days submerged from Europa to the US.

Therefore I conclude that even the smallest German AIP submarines, the Type 212 could stay for more than 3 weeks submerged without traveling.

That is game changing because it will make hunting submarines far more difficult.

As you mentioned. Not the AIP makes the great difference. It is the crew, training and maintenance. During Falklands War the Argentine Navy operated 3 submarines. One was out of order, one rather old and the last not in good conditions. The Royal Navy was lucky to encounter such a submarine "fleet".


Pete said...

Hi MHalblaub

No doubt the Argentinian submariners had a healthy fear of the British SSNs in the area.

This should also be a concern for Australians who only foresee an SSK only Australian future submarine fleet. SSKs are outclassed by SSNs under many or most mission scenarios.